1. Field of the Invention
The present invention relates to an electrically conductive synthetic resin typically used for buildings, vehicles and aircraft, as well as for industrial and agricultural purposes.
2. Description of the Related Art
In general, typical synthetic resins have heretofore been utilized as electrical insulators having a volume resistivity of the order of 10.sup.10 .OMEGA./cm.sup.3. However, such a conventional type of synthetic resin tends to be electrostatically charged because of its electrical isolating properties. This may lead to various problems; for example, dust may be electrostatically attracted to the surface of such a synthetic resin, the breakdown of an IC may take place, or, occasionally, an explosion might be caused by spark discharge derived from static electricity.
Also, electromagnetic wave noise propagated through air may invade computers or other electronic machines. It is of course necessary to prevent such invasion of unwanted waves. During the period in which electronic machines were typically made of metal, no problems were experienced due to the invasion of electromagnetic wave noise. This is because metal generally has a good shielding effect with respect to electromagnetic waves. However, with the development of integrated circuits and multilayer printed-circuit boards, the demand has risen for further progress in developing electronic devices offering compactness, lightness and adaptability to mass production. In response to such demands, certain kinds of synthetic resin have been utilized for the housings of such electronic devices, and this may result in the occurrence of various problems derived from electromagnetic wave noise.
For this reason, there has been an increasing demand for the advent of synthetic resins having proper electrical conductivity, suitable properties with respect to the prevention of electrostatic charging, and the capability of shielding electronic devices from electromagnetic wave noise. In general, synthetic resins have corrosion resistance, lightness, transparency, good formability and other characteristics which cannot be achieved with metal materials. In a variety of industrial fields, there is a strong demand for the development of a synthetic resin having both the characteristics described above and electrical conductivity of a quality approximately equal to that of metal.
Two types of electrically conductive synthetic resin have heretofore been known; one is of the type constituted by high polymers that themselves have electrical conductivity and the other is of the type constituted by high polymers mixed with finely divided metal or carbon and having an electrically conductive coating, flame-sprayed zinc coating, metal film formed by galvanization, deposited metallic foil or the like for the purpose of providing electrical conductivity.
However, the current techniques have not yet succeeded in imparting a satisfactory level of electrical conductivity to the first of these known types, i.e., an electrically conductive synthetic resin of the type that includes high polymers which themselves have electrical conductivity.
The latter type of electrically conductive synthetic resin is obtained through certain steps of specially treating metal such as silver, copper or aluminium; dividing the thus-obtained metal in the form of powder or flakes; and mixing the powdered or flaked metal with vinyl chloride, poly ethylene or the like by dispersion. In the case of such an electrically conductive synthetic resin, the volume resistivity is in the order of 10.sup.0 to 10.sup.-6 .OMEGA./cm.sup.3 at the best. In order to improve such electrical conductivity, an enormous amount of conductive material may be mixed with polymer by dispersion to provide close contact between the particles of the conductive material. However, this treatment nullifies the aforesaid characteristics that are inherent in a synthetic resin, thus resulting in a deterioration in the formability and mechanical strength thereof. There is another problem in that uniform electrical conductivity cannot be achieved, the extent of this problem depending upon the forming conditions.
Also, the aforesaid electrically conductive coating is typically formed by dispersing fine particles (2 to 3 micron in diameter) of nickel, silver or copper in a resin binder. Production of an electrically conductive synthetic resin covered with such an electrically conductive coating is relatively inexpensive as viewed in terms of plant and equipment as well as in terms of production costs. In addition, such electrically conductive coating is advantageous in that it requires no pre-treatment, in that it can be subjected to natural drying, and in that it is readily applicable to a relatively complicated configuration; accordingly, the aforesaid electrically conductive coating is currently used in a great number of fields. However, in general, electrically conductive coatings may exfoliate or crack over a long period of time, with the result that the shielding effect with respect to electromagnetic waves might be weakened. Also, since a certain period of time is required for drying, it is impossible to provide a mass production line, and this leads to the problem that the production involves a large amount of manual work.
The aforesaid electrically conductive synthetic resin with a flame-sprayed zinc coating is advantageous in terms of electrical conductivity but requires treatment at high temperatures. This may lead to the problem that the synthetic resin product becomes curved.
The aforesaid electrically conductive synthetic resin coated with a metal film formed by galvanization or deposited metallic foil may lose its shielding effect with respect to electromagnetic waves due to exfoliation or the formation of cracks as in the case of the aforementioned electrically conductive coating. This may likewise result in the problem that a mass production line is difficult to constitute.